In his French patent application No. 82 01589 filed on the Feb. 1, 1982, the applicant referred to the considerable economic interest in collecting microalgae or other living micro organisms. The cost of the usual processes for concentrating the liquids which contain them and for recovering these particles from the concentrations thus formed is unfortunately very high. Furthermore, the addition of chemical reagents for flocculating the desired products risks causing a deterioration of the qualities making these products interesting.
To overcome these drawbacks, the applicant in the above mentioned patent application has proposed an original process for flocculating particles in suspension in a liquid, which does not require chemical reagents and which can be used at high yield, without involving costly operations or apparatus.
The inventor there in fact discovered that a suspension of colloidal particles passing through a granular medium causes a more or less irreversible flocculation of the particles, affecting a variable fraction of the flow treated depending on the nature of the particles. This phenomenon occurs in particular in a way which may be readily observed with microalgae suspensions or else with effluents from biological waste water purifying units (tertiary filtration).
In the above mentioned patent, a first liquid comprising microscopic particles in suspension is caused to pass through a granular medium until said granular medium is partially or completely clogged by said microscopic particles, said granular medium is at least partially unchoked by causing a second liquid to pass therethrough flowing at a higher speed than the first one, and, downstream of said bed in the direction of passage of said liquids, flocculated aggregates of said microscopic particles and a liquid effluent are recovered, partially or totally free from said particles.
The second liquid may be identical to the first liquid and contain particles in suspension. The first and second liquids pass advantageously through the granular medium from bottom to top and this granular medium may remain in a fixed bed or undergo an expansion of about 30% in volume at most during implementation of the process.
The mechanism of the above mentioned process may therefore be summed up in the following way:
from a certain choking up rate of the granular medium, the microscopic particles self-flocculate without the help of any chemical agent, within this granular medium;
during the unchoking phase, the aggregates thus formed are carried along by the liquid and freed from the granular medium on the surface of which they decant on condition, of course, that the upward speed of the liquid is less than the speed at which these aggregates are carried away.
In continuous operation, a slight expansion of the granular medium is maintained in a way such that it presents sufficient resistance to the passage of the particles so that they flocculate and such that, simultaneously, this medium is sufficiently permeable for the aggregates thus formed to be able to pass therethrough and be released.
To this end, in practice, as is described in the prior patent application, the partially purified liquid effluent is recycled, at least partially, to the supply side of the granular medium.